1. Field of the Invention
This invention relates to a method of manufacturing main plates for exposure printing, and particularly to a method of manufacturing a pair of main plates for printing a predetermined pattern on photo-sensitive layers respectively formed on both faces of a shadow mask of a color cathode ray tube when the shadow mask is manufactured by a photo-etching method.
2. Description of the Related Art
A color cathode ray tube of shadow mask type is provided with a panel having a substantially rectangular face plate and a skirt extending from a peripheral edge of the face plate, a funnel connected to the panel, and a neck continuous to the funnel. The panel, funnel and neck holds the interior of the color cathode ray tube in vacuum. Within the neck is housed an electric gun assembly which generates electron beams. A deflection yoke which generates magnetic fields is mounted on the outer lateral surfaces of the funnel and the neck. A phosphor screen is formed on the inner face of the face plate of the panel. In the tube, a substantially rectangular shadow mask is disposed to be separated from the face plate at a predetermined distance so as to face the phosphor plate. The shadow mask is made of a thin metal plate and is formed with a great number of apertures. A mask frame surrounds the shadow mask. A plurality of elastically deformable mask supports are welded to the frame. Stud pins engaged with the supports are provided on the inner face of the panel.
With the color ray tube of shadow mask type, the three electron beams emitted from the electron gun assembly are deflected horizontally and vertically by the magnetic field generated by the deflection yoke, and thereafter are converged into apertures of the shadow mask. The electron beams converged at the apertures of the shadow mask are landed on the phosphor screen formed on the face plate of the panel. The phosphor screen has three kinds of phosphor stripes which are alternately arranged. When these phosphor stripes are shot by the three electron beams passing through the apertures of the shadow mask, these phosphor stripes emit three color lights of red, green and blue. In other words, the apertures of the shadow mask serve to direct the three beams to the predetermined phosphor stripes which respectively emit red, green and blue lights.
The shadow mask has a great number of apertures which open at the side of the phosphor screen and at the side of the electron gun. The openings of the apertures at the side of the phosphor screen are larger than the openings of the apertures at the side of the electron gun, and the approximately middle portion of the holes has the minimum diameter, such that each aperture has a so-called drum shape.
The apertures of the shadow mask are formed by the photo-etching method. The shadow mask is made of a plate-like shadow mask material of low carbon steel or the like. Photo-sensitive layers are formed on both the faces of the shadow mask material. A pair of main plates each formed with a pattern corresponding to the apertures of the shadow mask are closely contacted with the respective photo-sensitive layers, and thereafter these layers are exposed to light, whereby the patterns of the main plates are printed on the photo-sensitive layers on both the faces of the shadow mask. After the exposed photo-sensitive layers have been developed, the unexposed portions are removed from the photo-sensitive layers. Accordingly, resist layers which have patterns corresponding to the patterns of the main plates are formed on the shadow mask material. Then, the shadow mask material is etched at its both faces to form apertures which penetrate the shadow mask material.
In order that apertures are formed by etching both the faces of the shadow mask material as described above, the centers of each pair of the etched concave portions in both the faces of the shadow mask, which are deepened and finally merge, must be aligned. In view of the manufacturing technique, however, it is extremely difficult that the positions of the patterns formed on each main plate coincide with each other over the overall shadow mask.
A pair of main plates will be manufactured as follows:
FIG. 1A shows a method of manufacturing a main plate to be closely contacted with the face of a shadow mask material at the side of the phosphor screen, and FIG. 1B illustrates how to fabricate a main plate to be closely contacted with the face of the shadow mask material at the side of the electron gun.
As shown in FIG. 1A, a pattern 24 for etching the face of the shadow mask material at the side of the phosphor screen is drawn by a pattern drawing device on a negative dry plate 22. The dry plate 22 is developed to form a first negative pattern plate at the side of the phosphor screen. Next, a similar negative dry plate 26 is closely contacted with the first negative pattern plate at the side of the phosphor screen and printed. By developing the dry plate 26, a positive pattern 28 is formed. Thus, a positive pattern plate having the positive pattern 28 is manufactured which is used as a master. Further, a negative dry plate 30 is closely contacted with the positive pattern plate and printed, and the development of the negative dry pattern 30 creates a pattern 32, whereby a second negative pattern plate having the pattern 32 at the side of the phosphor screen is formed which is used as a first main plate for forming the apertures of a shadow mask.
A pattern 36 for etching the face of the shadow mask material at the side of the electron gun is drawn by the pattern drawing device on a negative dry plate 34 as shown in FIG. 1B. The dry plate 34 is developed to form a first negative pattern plate at the side of the electron gun. Next, a similar negative dry plate 38 is closely contacted with the first negative pattern plate at the side of the electron gun and printed. By developing the dry plate 38, positive pattern 40 is formed, whereby a positive pattern plate is manufactured which is used as a master. A negative dry plate 42 is closely contacted with this positive pattern and printed. The dry plate 42 is developed to form a pattern 44, whereby a second negative pattern plate having the pattern 44 at the side of the electron gun is formed which is used as a second main plate for forming the apertures of a shadow mask.
There are two reasons why each pair of main plates are manufactured by the above-described method.
First, since the image of the first negative pattern plate is inverted and the positive pattern plate is produced, inequality of patterns can be easily found which might be caused by such defects as small deviation in pattern dimensions and irregularity of the patterns. It should be noted that negative patterns formed by the pattern drawing device have a high light transmittance, making it difficult to find the above-mentioned defects, whereas the above-described positive patterns have a low light transmittance, making it relatively easy to find the above defects.
Secondly, when the positive pattern plates are used as masters, a great number of main plates can be directly produced.
With the shadow mask of a conventional color cathode ray tube used for a commercial purpose, apertures with a required shape and accuracies can be formed by means of a pair of main plates made by the above-mentioned method. Since, however, with the shadow mask of a color cathode ray tube such as display tube, in which a high resolution is required, the apertures are small and arranged at a small pitch. Accordingly, it is difficult to form apertures with a required shape and accuracies in main plates manufactured by the above method. It is because positional displacement tolerances are small for a pair of main plates. In other words, it is because a small positional displacement tolerance is allowed between the pattern on the shadow mask at the side of the phosphor screen which corresponds to the pattern of the first main plate bearing the apertures at the side of the phosphor screen, as one party, and the pattern on the shadow mask at the side of the electron gun which corresponds to the pattern of the second main plate bearing the apertures at the side of the electron gun, as the other party. For this purpose, the deviation of alignment between those etched concave portions in both the faces of the shadow mask, which are formed from the patterns of the two main plates must be less than 3 microns. A factor which causes such displacement between the patterns of a pair of main plates is that a problem exists in the way in which the patterns are described on a first dry plate by a pattern drawing device when the paired main plates are manufactured. In more detail, when a pattern drawing device is used, patterns are drawn in the same direction of the order of the rows of the patterns, that is, from the first row to the n'th row (n&gt;1) on a pair of dry plates. When the patterns are drawn on dry plates at an angle even slightly different from the angle required for the pattern, the patterns of the paired dry plates are drawn similarly in a state inclined from the required angle due to the function and the drawing accuracy of the main pattern drawing device. In this case, the positional displacement between the upper end and the lower end of the patterns in the outermost n'th row of the shadow mask for a 20-inch color cathode ray tube, for example, is 5 to 7 microns. Patterns are drawn by the main pattern drawing device on both dry plates with their emulsion faces disposed upward, and thereafter both dry plates are placed on the other dry plates such that their emulsion faces are directed to those of the other dry plates, and the patterns of both the dry plates are transcribed to the other dry plates to form positive pattern plates each having a positive pattern. The pattern of the positive pattern plate is also transcribed to further dry plates, thereby manufacturing a pair of main plates.
FIGS. 2 and 3 show a pattern formed on a main plate closely contacted with a shadow mask material when a shadow mask is manufactured. When a pair of main plates are used for printing a shadow mask, the emulsion faces formed with patterns are directed to the photo-sensitive layers of the corresponding faces of a shadow mask material and are closely contacted therewith. Upon printing the patterns on the shadow mask material, the first and second main plates are directed to the corresponding faces of the shadow mask material and placed thereon such that the pattern in the first row on the first main plate and the pattern of the last row on the second main plate form, on the shadow mask material, apertures which coincide with each other. If the rows of the patterns to be formed on the shadow material are inclined from the required angle, the inclinations of these rows of the patterns on the first and second main plates are reversely directed as shown in FIG. 2, because the two patterns face the corresponding faces of the shadow mask material. As a result, each pair of patterns on the shadow mask material are greatly displaced from each other as shown in FIG. 3.